The objective examination of intraocular scattering, which occurs when light interacts with ocular structures before forming the image on the retina, is particularly important for improving the early diagnosis of some of the most widespread ocular pathologies, particularly cataracts, which is the pathology that causes the highest rate of blindness worldwide, but also dry eye syndrome, corneal edema, etc.
Optical quality of the retinal image is the first determining factor of visual capacity. Like any optical system, the study of the eye in terms of its optical quality is approached by means of analyzing geometric characteristics, such as the shape of the cornea and the lens or the axial length, and also by analyzing the intrinsic properties of the ocular media with which light interacts on its way to the inner portion of the eye. In relation to the geometric characteristics of ocular surfaces, wavefront sensors (Prieto et al., 2000) evaluate aberrations of the eye, and therefore optical quality of the eye can be represented through PSF (Point Spread Function). The form of this function is determined by the geometric quality of ocular surfaces, the refractive index gradient and the axial length of the eye. The difference between an ocular PSF and the theoretical form of the function, which is obtained by assuming that the eye is a perfect optical system limited only by diffraction, relates to the induction of aberrations of the eye. However, although the intraocular scattering produced by the non-uniformity of the ocular media also heavily determines the quality of the retinal image, the characterization thereof is not included in the description of optical quality in terms of aberrations of the eye (Díaz-Doutón, Benito, Pujol, Arjona, Güell, & Artal, 2006). Intraocular scattering originates from the interaction of light with non-uniformities in the refractive index distribution of the ocular media, and it causes the light to scatter on the fundus of the eye, instead of directing the focused light to the central retina. The extension of this light scattering is described using statistical terms. Intraocular scattering can have a serious impact on vision, especially when natural scenes with the presence of bright sources are observed. An example is standard night driving conditions. Scattered light inside the eye causes retinal image contrast to decrease, and this therefore results in a serious reduction of vision quality.
Though treated separately due to the different causes producing them, aberrations of the eye and intraocular scattering both affect retinal image quality. The double-pass technique (Santamaria, Artal, & Bescos, 1987) based on projecting a collimated beam onto the patient's retina and directly recording the light reflected back allows characterizing the effect of aberrations of the eye and intraocular scattering on the point source projected onto the retina. Information on intraocular scattering is, however, restricted to analyzing the recorded retinal area, which is generally less than 1% the field of view. In current clinical use of the double-pass instrument (US 2010/0195876, 2008; Artal et al., PLOS One, 2011), information relating to intraocular scattering is estimated by means of analyzing the intensity recorded in the periphery of the double-pass image with respect to the total intensity of the PSF. The limitation of this technique lies in the fact that since the intensity of PSF drops rapidly with the angle from the central maximum to the most eccentric areas, only the smallest angles can be evaluated. Beyond a half degree of eccentricity, the intensity of the light in the PSF of a normal eye is so low that it is impossible to discriminate the signal from the background, and therefore only scattering at small angles can be evaluated. Furthermore, most clinical instruments used today use infrared light to generate the point source on the retina, the image of which is subsequently analyzed to estimate the intraocular scattering produced. Although this characteristic is suitable for minimizing the discomforts of a visible light beam visible for the subject, it is not optimal for obtaining a good estimate of intraocular scattering. The interaction of infrared light with the fundus of the eye produces a high level of scattered light, and although this does not affect visual capacity, it does contribute to estimating light scattered onto the double-pass image. Considering these limitations, the standard technique for analyzing double-pass images to estimate intraocular scattering is restricted to analyzing a limited region around peak or central maximum of the recorded image. This limitation in turn means that the presence of aberrations of the eye, modifying the distribution of intensity on the double-pass image also in this zone closest to the peak, could lead to interpreting an increase in the intensity around the peak that was actually produced by the effect of aberrations of the eye as scattered light. Although the effect of low order aberrations, such as defocusing, can be minimized to restrict the analysis of double-pass images, the contribution of high order aberrations cannot be completely eliminated in a clinical setting. Therefore, the estimate of intraocular scattering based on the double-pass technique with a point source seems particularly suitable for those cases in which the amount of scattered light is relatively high, like in a cataract process or in severe dry eye syndromes, but it does not seem suitable for general use as a technique for estimating the level of scattered light in normal eyes or in eyes with incipient pathologies occurring with a progressive increase in intraocular scattering.
Similar limitations are also shared by methods designed to characterize intraocular scattering from analyzing images recorded with a Hartmann-Shack sensor (Thibos & Hong, 1999). The dynamic range of the images analyzed in this case is also too low to enable discriminating scattered light above the fundus level intrinsic to the measurement. Other objective techniques for estimating intraocular scattering have been proposed, such as measuring dynamic scattering (Datiles, Ansari, & Reed, 2002; Vivino, Chintalagiri, Trus, & Datiles, 1993), but none of these techniques is suitable for being used in clinical environments; in fact, up until now none of these techniques had been implemented in a clinical instrument.
On the other hand, there are other systems that seek to estimate the magnitude of scattered light in the eye from psychophysical, and therefore subjective, measurements. Some examples are the visual acuity test designed by Holladay (Holladay, Prager, Trujillo, & Ruiz, 1987), which evaluates the drop in visual acuity produced by the presence of glaring sources in the field of view. The patient looks at the acuity test through a hole in a hemisphere. Illumination of the inner face of this hemisphere can be externally controlled by the operator such that changes in visual acuity of the subject caused by increasing levels of intensity in the light inside the hemisphere can be measured. Other subjective methods are also based on examining the effect of glaring sources on different visual functions such as contrast sensitivity (Bailey & Bullimore, 1991). Another psychophysical method is the direct compensation method based on the presentation of a glaring ring-shaped source with oscillating intensity and the compensation of this effect in the fovea through the control of a central source the intensity of which oscillates in counterphase to the glaring ring-shaped source. This method was implemented in the stray-light meter (van den Berg & IJspeert, 1992). An improved version of this methodology, the compensation comparison method (Franssen, Coppens, & van den Berg, 2006), was subsequently proposed. This device has been used in different applications related to estimating intraocular scattering (van den Berg, et al. 2007). The fundamental limitation of this instrument is the same as with other subjective systems because it requires the active participation of the subject in the measurement process. In this device, the participation requirement is especially demanding because the compensation comparison method entails getting the subject involved in a sequence of forced decisions related to the relative luminance of two rapidly changing central semi-fields. This process of forced, consecutive and rapid decisions obviously involves problems for many subjects, particularly for those in whom the reliable estimate of intraocular scattering is particularly relevant, as in elderly subjects.
Until now, there has not been any optical instrument capable of suitably measuring the intensity of scattered light in normal (non-pathological) eyes from a purely optical measurement, i.e., from an objective technique and without requiring active intervention of the subject. Therefore, there is a need for an optical instrument capable of carrying out a method for objectively measuring intraocular scattering at large field of view angles, for example up to 10°. Furthermore, it is especially relevant that this system allows measuring using different wavelengths, because the dependence of scattered light with incident wavelength is strongly related to the typology of the non-uniformities created by the scattering. Therefore, being able to access this characterization with the wavelength may provide relevant information concerning the underlying pathology.